Abstract

We have demonstrated that the ion current resulting from collisions between metastable krypton atoms in a magneto-optical trap can be used to precisely measure the trap loading rate. We measured both the ion current of the abundant isotope Kr83 (isotopic abundance=11%) and the single-atom counting rate of the rare isotope Kr85 (isotopic abundance∼1×10−11), and found the two quantities to be proportional at a precision level of 0.9%. This work results in a significant improvement in using the magneto-optical trap as an analytical tool for noble-gas isotope ratio measurements, and will benefit both atomic physics studies and applications in the earth sciences.

Figures (5)

Four biased electrodes (E1–E4) guide the Kr+ and Kr2+ ions, produced by collisions between trapped atoms (red dot), toward the Faraday cup. (a) Schematics of the setup. (b) SIMION calculations of the electric potential (red curves) and simulations of the ion trajectories (blue lines). The thick patch of blue lines is formed by a range of trajectories due to the initial distribution of positions and momenta. The initial kinetic energy of the Kr ions is set to be 5 eV, which is close to the maximum kinetic energy the Kr+ and Kr2+ ions can get in the ionization processes.

Kr83 ion current versus the number of trapped atoms. The dashed line corresponds to the simple N2 dependence. η is the ion collection efficiency; β is a coefficient defined in Eq. (1). The solid line shows the results of the Fermi function model. Inset: density distributions of atoms in the trap. Various curves correspond to different total numbers of atoms in the trap.

Kr83 ion current versus Kr85 loading rate. Both are varied deliberately by more than 1 order of magnitude. The proportional relationship proves that the ion current is a robust measure of the trap loading rate.

Kr85/Kr83 versus Kr85 activities (low-level decay counting). The standard units for Kr85 activity is dpm/cc: decays per minute per cubic centimeters of Kr gas at standard temperature and pressure (100dpm/cc corresponds to an isotopic abundance of 3×10−11). The agreement between the results measured by two completely different methods provides further validation of the ion-current method.